Method of preventing etch in plating baths

ABSTRACT

THIS INVENTION RELATES TO NOVEL COMPOSITIONS AND TO THE PROCESS FOR ELECTROPLATING A PLATE METAL ONTO A CATHODE OF HIGH HYDROGEN OVERVOLTAGE BASIS METAL HAVING AREAS OF LOW CATHODIC CURRENT DENSITY WHEREON PLATING DOES NOT OCCUR AND SELECTED AREAS OF HIGH CATHODE CURRENT DENSITY WHICH COMPRISES PLACING ONLY THE CATHODE AREAS OF LOW CATHODE CURRENT DENSITY IN ITIMATE ELECTRICAL CONTACT WITH A LOW HYDROGEN OVERVOLTAGE METAL HAVING A HYDROGEN OVERVOLTAGE OF LESS THAN ABOUT 0.3 VOLT AT A CURRENT DENSITY OF 0.1 AMPERE PER SQUARE DECIMETER; AND PLATING THE PLATE METAL ONTO SAID SELECTED AREAS OF THE SURFACE OF SAID CATHODE, THE SAID CATHODE AREAS OF LOW CURRENT DENSITY REMAINING SUBSTANTIALLY FREE OF ETCHING DURING SAID PLATING.

United States Patent 3,702,809 METHOD OF PREVENTING ETCH 1N PLATING BATHS Ram Dev Bedi, Southfield, Mich., assignor to M & T Chemicals Inc., New York, N.

No Drawing. Continuation-impart of application Ser. No. 221,879, Sept. 6, 1962. This application May 1, 1969, Ser. No. 821,135

Int. Cl. C231) 5/48 US. Cl. 204-15 22 Claims ABSTRACT OF THE DISCLOSURE This invention relates to novel compositions and to the process for electroplating a plate metal onto a cathode of high hydrogen overvoltage 'basis metal having areas of low cathodic current density whereon plating does not occur and selected areas of high cathode current density which comprises placing only the cathode areas of low cathode current density in intimate electrical contact with a low hydrogen overvoltage metal having a hydrogen overvoltage of less than about 0.3 volt at a current density of 0.1 ampere per square decimeter; and plating the plate metal onto said selected areas of the surface of said cathode, the said cathode areas of low current density remaining substantially free of etching during said plating.

This invention relates to plating and more particularly to a technique for preventing etching on cathodic areas of low current density. This application is a continuationin-part of application Ser. No. 221,879, filed Sept. 6, 1962, now abandoned.

As is Well known to those skilled in the art, electroplating of various metals, typically nickel, copper, tin, zinc, or chromium may be effected in baths of varying acidity and composition; many of these baths may contain chloride or fluoride together with oxidizing agents Which may be an integral part of the bath or present as an additive. During plating of basis metals in such baths, it is necessary to control current density over the areas to be plated. Because of difierences in area or geometry or accessibility, there are portions of the cathode which have a current density low enough so that plating is not expected to and in fact does not occur. It has been found, particularly when the bath is acid and contains e.g. chloride or fluoride ions together with oxidizing agents, that in these low current density areas, etching is a problem. Specifically, the surface of these areas may be corroded during plating operations sufiiciently to alter the dimensions substantially and/ or to spoil the appearance of the finished article. The problem of etching is present in various baths, typified by chromium plating baths, nickel plating baths, acid copper baths, acid tin baths, acid zinc baths containing halides or halide-complexes and frequently containing oxidizing agents, etc.; for purposes of convenience, reference will hereinafter be made to chromium plating baths.

As is well known to those skilled in the art, chromium plating for example may be effected by use of a bath containing chromic acid and sulfate together with other compounds which may be employed to effect various desirable results. Typical of these additive compounds are fluorides or fluoride complexes.

Other illustrative chromium plating systems include soluble catalyst systems containing e.g. chromic acid and sulfate such as sulfuric acid in amount sufiicient to give the desired concentration of sulfate ion, or self-regulating baths, typically those containing silicofluorides together with sulfate.

During chromium plating from baths typified by the 3,702,809 Patented Nov. 14, 1972 ICC foregoing, it is common to operate at a temperature which may vary depending upon the concentration of CrOg. Commonly, however, the temperature of operation is 34 C.-72 C. and typically 48 C.63 C. The cathodic current density is preferably controlled to fall in the range of 8-60, and typically 12-30 amperes per square decimcter on the selected areas of high current density Whereon plating may occur. However, because of the irregular shape of many pieces which are to be chromium plated, it is not possible to maintain uniform desired current density over the entire piece. If the current be set to provide a current density as noted, there will he places, typically interior portions, end portions, back portions, or crevices where the current density may be considerably lower, for example 1.6 or less and frequently 0.3-1.0 ampere per square decimeter.

It has been found that these areas of low current density are strongly etched during plating and especially so when the bath is a chromium plating bath which contains fluoride or silicofluoride ions which may be present in selfrcgulating high speed baths, soluble catalyst baths, or sparingly soluble catalyst chromium plating baths. In order to eliminate this etching in chromium plating systems, it has heretofore been common to coat with tapes or waxes those portions of the cathode where low current density is anticipated. The use of such techniques is time consuming and expensive. Tapes or waxes must be carefully placed and removed, since any holes or spaces (including edge areas which may be undermined) in the protective covering will allow severe etching of the soexposed metal to take place. Such tapes or waxes may be expensive to purchase and apply. Various other attempts to minimize low current density etching of cathodes in the noted plating baths have not been uniformly success ful and there is today no economical etch preventive system which is completely satisfactory.

It is an object of this invention to provide a technique for preventing undesirable etching in plating operations including e.g. chromium and nickel plating operations. Other objects will be apparent to those skilled in the art on inspection of the following description.

In accordance with certain of its aspects, the process of this invention for electroplating a plate metal onto a cathode of high hydrogen overvoltage basis metal having areas of low cathode current density and selected areas of high cathode current density, comprises placing only the cathode areas of low cathode current density whereon plating does not occur in intimate electrical contact with a low hydrogen overvoltage metal; and plating the plate metal onto said selected areas of the surface of said cathode, the said cathode areas of low current density remaining substantially free of etching during said plating.

Typical of the plate metal with which the process of this invention may find use is the aforementioned chromium plate including hard chromium plate. This invention will be especially siutable for use with the noted chromium plating baths, including the soluble or sparingly solubl catalyst-containing chromium plating baths, since it is with these baths that the problems of etching may be most severe because of the presence of chloride, fluoride, or silicofluoride ions in the bath.

The cathodes which are used in the practice of this invention are those articles upon which a plate, typically a chromium plate is desired. Typically these basis metals are characterized by their high hydrogen overvoltage. Hydrogen overvoltage is a measurement of the amount of work required to liberate hydrogen at the cathode. Thus, it is conventionally expressed as the difference between the potential of the electrode when hydrogen is liberated during electrolysis and the potential of the reversible hydrogen electrode, both potentials being referred to the same electrolyte. A cathode having a lower hydrogen overvoltage will, therefore, generate hydrogen at a lower voltage than a cathode having a higher hydrogen overvoltage. High hydrogen overvoltage basis metals include ferrous metals such as iron or steel, including stainless steels, low carbon steels, nickel steels, chromium steels, chormium-nickel steels, etc., particularly when these metals are in bright, solid, highly polished conditions, and are defined as those metals which have a hydrogen overvoltage at least as great as ferrous metals. These basis metals are characterized by high susceptibility to etching at low current density areas during e.g. chromium plating especially where the baths employed are fluoridecontaining chromium plating baths.

In the practice of this invention the portions of the cathode on Which etching may occur are placed in intimate electrical contact, e.g. coated, with a low hydrogen overvoltage metal. Low hydrogen overvoltage metals, as the term is used in this application, include those metals which have a hydrogen or activation overvoltage less than that of the well known intermediate hydrogen overvoltage metals. Typically the metals which may be used in practice of this invention have an overvoltage equal to or less than the overvoltage of clean massive nickel. These low overvoltage metals have an overvoltage of less than about 0.3 volt at a current density of 0.1 ampere per square decimeter as defined for example in The Corrosion Handbook by H. Uhlig (John Wiley, 1948), page 1144. As will be apparent to those skilled in the art, the hydrogen overvoltage of a metal may be modified by the method of treating the metal surface.

Typical of such metals which may be used to provide low hydrogen overvoltage are platinum, palladium, rhodium, gold, iridium, and nickel including block nickel and Electroless nickel.

These protective metals, in form suitable to provide a low hydrogen overvoltage, are placed in intimate electrical contact with the surface to be protected. They may be employed in connection with the cleaned or prepared surface of the basis metal in a number of ways. One highly convenient method for applying the low hydrogen overvoltage metal to the areas of low cathode current density is by deposition, electrolytically, chemically, or by immersion, from a solution containing an ion of a low hydrogen overvoltage metal. This coating or deposition may be accomplished e.g. by spraying, contacting, brushing, dipping, electroplating, wicking, immersion plating by the well-known Electroless nickel technique, etc. When the ions of the low hydrogen overvoltage metal come into intimate contact with the basis metal, the former are reduced to chemically form a deposit of the metal on the surface of the basis metal; and such deposits may typically be in the form of finely divided metal, e.g. black metal such as black nickel.

Compounds of the low hydrogen overvoltage metals including salts, acids, etc. may also be employed in the process of this invention. For example, chloroplatinic acid solutions may be employed as a source of platinum ions; and chloroauric acid may be employed as a source of gold ions. Complex ions of the metals may also be used, e.g. the halide and cyanide complexes of gold, the AuO (aurate) ion, etc. Similar equivalent metal compounds may be employed. When the low hydrogen overvoltage metal may exist in more than one oxidation or valence state, any of these may be employed.

Typical ionic metal compounds which may be used to obtain low hydrogen overvoltage metal deposits which may preferably find use in the practice of this invention include palladium dichloride, chloroplatinic acid, platinum chloride, platinum diamino dinitrite, potassium chloroplatinate, potassium chlorplatinite, tetrammine platinous chloride, tetrammine platinous fluoride, palladium nitrate, rhodium chloride, iridium tetrachloride, chloroiridic acid, gold chloride, gold cyanide, etc. There may be employed other deposits including that identified as Electroless nickel (q.v. ASTM Spec. Tech. Pub. 265), Electroless gold (q.v. e.g. J. Electrochem. Soc. 108, 767 (1961)), Electroless palladium (q.v. e.g. J. Electrochem Soc. 108, 707 (1961)), or Electroless platinum. The preferred compounds include palladium chloride, gold chloride, and chloroplatinic acid in aqueous solutions. Solutions of these in organic solvents such as ethanol, propanol, acetone, benzaldehyde, ether, etc. may be employed.

In a preferred embodiment, when it is desired to chromium plate a steel piece without etching low current density areas, these latter areas are coated with e.g. palladium by brushing or by dipping the piece into an aqueous (or a non-aqueous) solution of a palladium salt complex e.g. the chloride PdClf. Typically an aqueous solution containing at least about 0.01 g./l. up to the limit of solubility, and typically 3 g./l. of complexed palladium metal added as chloride, and 4-3 50 g./l., preferably g./l. of sodium chloride is used. The basis metal is painted with this solution which is allowed to stay thereon for typically 15-30 seconds. During this period, finely divided palladium metal coats the basis metal.

When the basis metal has acquired a coating of low hydrogen overvoltage metal, the solution of the latter is removed as by washing or rinsing with water, for example; the so-treated cathode is then ready for further processing.

In another embodiment, those portions of a steel cathode which when plated may be areas of low current density are dipped into a solution of auric ions e.g. auric chloride (having a concentration of at least about 0.01 g./l. up to 25 g./l., and preferably in amount of 4 g./l.) and immersed for at least about 12 seconds, after which time the cathode is removed from the solution, rinsed, cleaned in the usual manner, and placed in the plating tank.

When the desired thickness of plate metal, e.g. hard chromium plate, has been achieved, the cathode is removed from the bath; the low current density areas which had been treated by the process of this invention will be found to be substantially free of undesirable etchmg.

In accordance with another embodiment of the invention, a cathode (typically of steel) is treated by brushing or spraying thereon a solution of the low hydrogen overvoltage metal salt, and allowing the solution to remain in contact with the surface of the cathode for a period of time sufificiently long (typically l2 seconds) to insure adequate reaction.

In accordance with another embodiment of this invention, a novel etch-preventative composition for preventing etching on a low current density area of a high hydrogen overvoltage metal cathode comprises a compound of a metal which metal in finely divided form has a low hydrogen overvoltage and a substantial excess of a paste-forming vehicle. Typical paste-forming vehicles include pastes derived from water-soluble polymers (including their derivatives), such as starches, gums, carboxymethyl cellulose, pectin, alginates, polyvinyl alcohol, polyacrylamide, gelatin, clay (typically kaolin or bentonite), silica gel, caragheenates, etc. Other Wellknown equivalent paste-forming materials or thickening agents may be employed. The so-prepared paste is then allowed to remain in contact with the cathode until reaction therewith is achieved, and the residual paste is then removed by rinsing, rubbing, brushing, scrubbing, etc. When the paste method of treatment is employed, it will be particularly highly desirable to remove substantially all the residual material before plating to avoid poisoning the low hydrogen overvoltage property of the etch preventative deposit.

The immersion or contact time required to obtain adequate protection for the low current density areas of the cathode will be largely dependent upon the concentration of the metal in the treating solution or paste, the temperature at which treatment is carried out, the

nature of the basis metal, etc. Typically, the contact time will be of the order of at least 1-2 seconds. Shorter contact times may be employed, but the protection obtained thereby may not be as satisfactory. Longer contact times may also be used, although often no attendant advantages are enjoyed by substantially longer periods.

The treating solutions or pastes which may beemployed in the practice of this invention will be solutions or pastes containing a soluble ion of a metal which forms a deposit of low hydrogen overvoltage metal. Any concentration of the metal ion may be used from extremely low concentrations up to saturated solutions. However, if lower concentrations are employed, longer contact times and larger amount of solution or pastemay be required. If highly concentrated solutions or pastes are used, serious losses of the low hydrogen over-voltage metal may be suffered from drag-out. In order to produce a treating solution with the best balance of treating properties, it is preferable to employ the low hydrogen overvoltage metal in the equivalent metal concentration of at least about 0.1 g./l.

It is a feature of this invention that the etch prevention is accomplished by placing the low hydrogen overvoltage metal in intimate electrical contact in the solution with the cathode areas of low current density. This low hydrogen overvoltage metal may be provided by use of a thin foraminous sheet, including mesh, expanded metal, perforation metal, etc. in intimate electrical contact with, and preferably positioned immediately adjacent to and electrically connected to, the cathode areas to be protected. The foraminous sheet may be made of the low hydrogen overvoltage metal or it may be metal, e.g. steel, coated with a low hydrogen overvoltage metal such as platinum or palladium in the manner noted supra. The portion of the cathode to be prevented from etching is preferably overlaid with and contiguous to the thin foraminous sheet. Use of such a foraminous sheet immediately adjacent to and electrically connected to the cathode permits plating of the cathode with-minimum etch in the low current density areas. I

The etch-preventive quantity of the low hydrogen overvoltage metal which is required to elfectively protect the low current density areas of the cathode may be very small. Larger amounts may be employed, but it has been found that such larger amounts, i.e. thicker layers, will not appreciably be better for the prevention of undesirable etching. Although the preferred amount of low hydrogen overvoltage metal is approximately the amount required to form a monomolecular layer thereof on the area treated, "it may be found that a continuous monomolecular layer is notformeddnstead, the low hydrogen overvoltage metal may be present in the form of discrete islands and a considerable percentage of the treated area of the cathode may he apparently exposed to contact with the plating solution; It would be expected that such exposed areas would not be protected and would be etched in the same manner as a totally untreated cathode. Surprisingly, it has been found that even these relatively non-uniform treatments are highly effective in substantially eliminating the undesirable etching of the entire treated surface of the cathode.

The protective film or deposit of low hydrogen overvoltage metal possesses good adhesion to the cathode surface, and the treated cathode may subsequently be exposed to cleaning procedures before plating. When very thin films of low hydrogen overvoltage metal are used,e'.g. a 'suflicient amount to produce a monomolecular layer or less, the treated surface may look substantially similar to an untreated surface of the same material. Sometimes the treated surface may appear somewhat darker and more lustrous than the untreated. 'If this be objectionable, then the low hydrogen overvoltage metal may be removed by simple buffing after the plating operation.

it has been found to be particularly desirable to remove the solution or paste from the deposited film, particularly when that film is of palladium. Preferably, this is effected by a rinse with warm-to-hot water.

A specific preferred solution which may be used in practice of this invention contains 4 g. of palladium chloride, 20 g. of sodium chloride, hydrochloric acid sufficient to lower the pH to about 1.5, and water to make up one liter.

Another specific solution which may be used in practice of this invention contains 2.5 g. chloroplatinic acid (H PtCl -6H O) and water to make up one liter.

Another solution which may be used in practice of this invention contains 4 g. chloroauric acid hydrate (HAuCl -2H O) and water to make up one liter.

Specific illustrative paste compositions suitable for use in practice of this invention include the following:

Components Parts by weight A Silica gel Rhodium chloride. 1. 5

Water 1, 000

B Silica gel 100 Chloroirldic acid 3 Water 1, 000

C Methocel brand of methyl cellulose. 20 Rhodium chloride l E Polyvinyl alcohol (Swift Gelvatol Brand) 100 Rhodium chloride 2 Water 1, 000

F Gelatin (Keystone type C-J-X) 60 Ghloroiridic acid 4.

Water 1, 000

Other pastes may be formulated from the other pasteforming agents and compositions hereinbefore noted.

The following illustrative examples more clearly point up the novel features of this invention.

EXAMPLE 1 A cylindrical piece of pipe made of type 1015 steel 23.8 centimeters long, 3.8 centimeters outside diameter, and 0.31 centimeter wall thickness was degreased by treatment in a vapor-phase degreaser, and further cleaned by aniodic cleaning in alkali, rinsing in water, dipping in 2% sulfuric acid, rinsing in water, and drying. An aqueous solution (at 25 C.) containing 4 g./l. of palladium chloride, 20 g./l. of sodium chloride, and hydrochloric acid suificient to lower the pH to about 1.5, was brushed over the inside of the pipe. The solution was rinsed ofi. The treated area had a slight grey cast and was somewhat more lustrous than the untreated area. The pipe as anode was then cleaned in a 200 g./l. chromic acid bath at room temperature at 14.4 amperes per square decimeter based upon outside area to be plated. The pipe was then made the cathode in the chromium plating bath containing 250 g./l. chromic acid, 1.25 g./l. sulfate ion (as sulfuric acid) and 2.50 g./l. silicofluoride ion (as potassium silicofluoride). The anodes were lead alloy electrodes containing 4% tin. The outside surface area of the pipe was 285 square centimeters and the current density was 49 amperes per square decimeter, based on outside area. The pipe was plated for 2hours While the bath was maintained at 57-5 8 C. At the end of this time, it was removed from the bath, rinsed, and dried. The interior surface of the pipe, which was low current density area and not chromium plated, was examined for evidence of etching. The treated portion of the inside of the pipe was found to be substantially free from attack.

A standard piece of pipe was processed in identical manner except that the inside of the pipe was not treated with palladium chloride. The inside of this control piece 7 was found to be badly etched and had wide etch bands, particularly adjacent to the ends thereof.

EXAMPLES 2-7 In order to study the efficacy of low hydrogen overvoltage metal treatment in reducing etching at various low current densities, the following comparative experiments were run.

Steel cathode strips of cold rolled steel were buffed and further cleaned by the following procedure:

(1) Hydrochloric acid (6 N) dip for 10-15 seconds;

(2) Water rinse;

(3) Anodic cleaning in an alkaline cleaning tank for 15 seconds;

(4) Water rinse;

(5) Sulfuric acid (2%) dip for 10 seconds;

(6) Water rinse.

These cathodes (except for one control sample which was not treated) were immersed for 5-10 seconds in a solution containing 2.5 g./l. chloroplatinic acid, H PtCl -6H O which was maintained at 21-24 C. After treatment, the desired exposed area (as noted infra in Table I) was marked off on each cathode, and the remaining surface was covered with the chromic acid-resistant stop-off tape sold by the Minnesota Mining and Manufacturing Co. under the trademark Tape 470, typical stop-off tape intended to protect areas of metal which may be subjected to etching conditions during plating. The plating solution employed in these examples was a chromic acid solution containing 237 g./l. chromic acid, 1 g./l. sulfate ion (supplied from sulfuric acid) and 2.02 g./l. silicofluoride ion (supplied from potassium silicofluoride). The plating solution was placed in a series of 1000 cc. beakers which were held in a constant temperature bath at a temperature of 43.5 C. which was maintained constant throughout the experiment. The anodes used were lead alloy containing 4% tin. The electrodes in all the baths were connected in series to ensure that the same amount of current would pass through each, and the current was maintained at 100 milliamperes. Electrolysis was maintained for one hour, after which the cathodes were removed, rinsed, dried, and weighed, and the weight loss during electrolysis was determined. In Table I, this weight loss is noted as the weight loss in milligrams per square decimeter of surface per hour of electrolysis, and is designated wt. rng/dmF/hr.

TABLE I Area Current Wt. exposed, density, loss. Wt., mg./ Example Condition rnfi amp/(1m. mg. dmfi/hr.

2 Untreated 0.194 0.516 58. 4 301 0. 145 O. 69 1. 1 7. 6 0. 163 0.62 0. 3 1. 8 O. 210 0. 48 2. 5 -12. 0. 226 0. 44 1. 7 -7. 0 0. 258 0.387 0.9 3.

It is seen from Table I that practice of this invention permits attainment of substantially complete etch prevention at low current densities. In fact, comparison of e.g. Example 5 with Example 2 (untreated) indicates that, at comparable current densities, reduction in etch of about 94% may be realized.

EXAMPLE 8 The procedure of Examples 2-7 was duplicated except that the cathodes were immersed for S-1O seconds in a solution of 4 g./l. of chloroauric acid hydrate, HAuCl -2H O, (in place of the chloroplatinic acid) and the sample was tested at 0.7 ampere per square decimeter.

EXAMPLE 9 The procedure of Example 8 was duplicated except that the cathodes were immsersed for 5-10 seconds in a solution of palladium dichloride the same as that of Example 1 and the sample was tested at 0.7 ampere per square decimeter. v EXAMPLE 10 The procedure of Example 8 was duplicated except that the cathodes were immersed for 5-10 seconds in a solution of platinum chloride the same as in Examples 2-7 and the sample was tested at 0.7 ampere per square decimeter.

The results of Examples 8, 9, and 10 are tabulated as follows, showing the loss of weight after etching (each noted value is the average of several experiments):

Loss, mg./ Example Metal dmfi/hr.

8 old 20. 1 9 Pallad1um 9. 0 10 Platinum".-. 6.1

The control sample of Example 2. lost 301 mg./dm. /hr. in comparative tests g.v.

A further series of tests was conducted wherein the coating was provided on a metal screen immediately adjacent to and electrically connected to the cathode. In

Example: Weight loss 1 mg./dm. /hr. 1 l 2.8

12 "1--.. 1.7. 13 (standard) 382 1 0f steel.

From Examples 11-13 it is observed that the coating of low hydrogen overvoltage metal prevents etching.

Although this invention has been described with reference to specific examples, it will be apparent that various modifications may be made thereto which fall within the scope of this invention.

I claim:

1. The process for electroplating a plate metal in an acid plating bath onto a cathode of a high hydrogen overvoltage basis metal having areas of low cathode current density whereon plating does not occur and selected areas of high cathode current density which comprises placing only the cathode areas of low cathode current density in intimate electrical contact with a low hydrogen overvoltage metal having a hydrogen overvoltage of less than about 0.3 volt at a current density of 0.1 ampere per square decimeter; and plating the plate metal onto said selected areas of the surface of said cathode, the said cathode areas of low current density remaining substantially free of etching during said plating.

2. The process of claim 1 wherein the cathode is steel and wherein the cathode areas in intimate electrical contact with the low hydrogen overvoltage metal are coated with said metal.

3. The process of claim 2 wherein the plate metal is chromium.

4. The process of claim 3 wherein the chromium is electrodeposited from an aqueous chromic acid bath containing sulfate ions and complex fluoride ions.

5. The process of claim 4 wherein the coating is applied by contacting said areas of low cathode current density with an ionic compound of a low hydrogen overvoltage metal and depositing said metal on the cathode.

6. The process of claim 5 wherein the low hydrogen overvoltage metal is platinum.

7. The process of claim wherein the low hydrogen overvoltage metal is palladium.

8. The process of claim 5 wherein the low hydrogen overvoltage metal is rhodium.

9. The process of claim 5 wherein the low hydrogen overvoltage metal is gold.

10. The process of claim 5 wherein the low hydrogen overvoltage metal is iridium.

11. The process of claim 5 wherein the low hydrogen overvoltage metal is an immersion nickel.

12. The process of claim 5 wherein the low hydrogen overvoltage metal is a black nickel.

13. The process of claim 6 wherein the coating is applied by contacting said area with an ionic compound of a low hydrogen overvoltage metal and depositing said metal on the cathode.

14. The process of claim 13 wherein said low hydrogen overvoltage metal is deposited by chemical reaction of said ionic compound.

15. The proces of claim 13 wherein said low hydrogen overvoltage metal is deposited electrolytically.

16. The process of claim 1 wherein the cathode areas in intimate electrical contact with the low hydrogen overvoltage metal, are coated with said metal.

17. The process of claim 16 wherein the coating is applied by contacting said areas with a solution of an ionic compound of said low hydrogen overvoltage metal and depositing said metal on said areas.

18. The process of claim 17 wherein said solution contains as said ionic compound, a member selected from the group consisting of palladium chloride, gold chloride, and chloroplatinic acid.

19. The process of claim 16 wherein the coating is applied by coating said areas with a paste comprising (a) an ionic compound of said low hydrogen overvoltage metal, and (b) a paste-forming vehicle; maintaining said References Cited UNITED STATES PATENTS 493,277 3/ 1893 Lugo 204-15 1,274,995 8/1918 Crombie 204-47 X 1,750,418 3/1930 McFarland 204-143 X 2,061,592 11/1936 iRapids 204-15 2,367,314 1/1945 Russell 204-15 2,433,687 12/ 1947 Durst 204-143 X 2,503,863 4/1950 Bart 204-26 X 2,689,215 9/1954 Bart 204-224 X 2,764,538 9/ 1956 Smart 204-29 2,812,297 11/1957 Stareck et a1 204-34 3,012,920 12/1961 Christensen et a1 156-11 2,978,390 4/ 1961 Atwater et al 204-46 OTHER REFERENCES A. G. Gray, Modern Electroplating, pp. 358, 360 (1953).

GERALD L. KA'PLAN, Primary Examiner US. Cl. X.R. 

